Originally posted by Positron Using a 100k with 100k resistor will severly lessen the high frequency response. The feedback circuit won't work on this problem. Look below at some figures of high frequency loss.

Consider a stage of gain of only 10, with 4pf grid to plate capacitance (two tubes in parallel so twice the gp capacitance) and no stray capacitance (very optimistic). I have computed (computer program) the -1db and -3db responses.

I presume you are refering to my suggested parallel 6SN7 Circuit?

I think you will need to check your computer programs a little.... They seem to be written by sciencce fiction writers.

Alternatively you need to change your logic, WRT NFB.

Quote:

Originally posted by Positron
1) with 100k volume control set to midpoint
-1db = approx 6khz
-3db = approx 14khz

Not sure how you arrive at that, discounting stray the REAL results are actually with the volume control at midpoint (worst case) -1db @ 43KHz & -3db @ 84KHz, something I feel is tolerable for a Linestage to be used with CD Sources.

The variation of HF Response from worst to best case is minimal, with the volume set fully open (best case) the -1db point is @ 44KHz and the -3db point @ 86KHz, virtually no change from the "worst case" position.

I actually choose the circuit and recommended precisely because of the way it bahaves WRT to capacitive loading on the volume control, plus it's comparably low output imedance without using cathode followers outside a feedback loop. And of course because keeping the feedback loop around one stage only minimises the blatant sonic problems caused by using loop feedback around muktiple stages.

It makes a pretty good linestage, both subjectively and objectively, though I personaly would prefer an S&B TX-102, against which it can be easily compared BTW....

Sayonara

corbato

20th March 2004 01:00 PM

2 Attachment(s)

And this for the linestage. Just curious, cause as of yet I'm drawn toward Thorsten Sahibís schematics. I really want to use those 6SL7ís and 6SN7ís

ThorstenL

20th March 2004 03:12 PM

Konnichiwa,

Quote:

Originally posted by corbato And this for the linestage. Just curious, cause as of yet I'm drawn toward Thorsten Sahibís schematics. I really want to use those 6SL7ís and 6SN7ís

This linesatge will have a gain of over 20db. Combined with that Phonostage we are taliking overall around 100db gain. That is 30db too much gain for the usual 0.2-0.5mV MC Pickups and 50db too much gain for the usual 2-5mV output MM Pickups. It will also be almost 20db too much gain for any CD Player.

This would mean that your volume control will always be near the left endstop, the preamp will be xcessively noisy and suffer from poor overload behaviour.

For Preamps I'd recommend to consider the following issues in planning gain:

1) A CD player will output eaks of 2.8V with an average level around 14db below this, giving around 0.5V RMS for program material with 14db headroom.

LP's will be cut in the "energy" range of the music with up to 14db above the rated 5cm/s @ 0dB and a "bogey" MM Pickup outputs 2.5mV @ 5cm/s with a "bogey" low output MC pickup at 0.25mV.

This means we can have levels with a similar peak/average ratio than CD and similar average loudness if our MM Phonostage has a gain of around 46db and our MC stage of around 66db.

Most modern amplifiers will have an input sensitivity of around 1V RMS for nominal output. Older Valve Amp's and some rather badly designed modern Valve Amp's have a lower input Voltage, very few Amp's (such as my preferred 2-Stage Amp's with medium to low Mu triode drivers and DHT outputs) require a little more input.

This suggests that our Linestage will most of the time be required to attenuate, to have a littlke extra gain can help in marginal combinations, hence my preferred 6db Linestage gain.

So, the overall for a full function preamp with MC Input and the volume full up should be around 72db (the CAT matches that exactly) and for the MM input 52db (the EAR 834P which is a MM Phonostage aimed at directly driving integrated Amplifiers at CD level BTW has pretty much exactly this level of gain), in practice even these values tend to be 6db too high, leaving 6db gain reserve in the Phonostage and 6db gainreserve in the linestage, based on 1V Amplifier sensitivity. Any higher gain levels will cause serious system matching problems.

Sayonara

Positron

20th March 2004 04:44 PM

Wow

This design reminds me of the 15,000pf IC that was going to be marketed. "Hey it sounds good". Of course it didn't and the frequency response of the preamp it was connected was -3db about 7khz. "But it sounded good".

"I presume you are refering to my suggested parallel 6SN7 Circuit?

I think you will need to check your computer programs a little.... They seem to be written by sciencce fiction writers.

(It is an engineering program, you are way off, either in calculating or the thevenin circuit itself.)

Alternatively you need to change your logic, WRT NFB.

(You obviously haven't thought things through. More on this below. Last string I mentioned there is a whole world you don't understand. You just demonstrated it again.)

Not sure how you arrive at that, discounting stray the REAL results are actually with the volume control at midpoint (worst case) -1db @ 43KHz & -3db @ 84KHz, something I feel is tolerable for a Linestage to be used with CD Sources.

(You better check your math again, or get the thevenin circuit correct. I also didn't include the capacitance of the volume control itself either.)

The variation of HF Response from worst to best case is minimal, with the volume set fully open (best case) the -1db point is @ 44KHz and the -3db point @ 86KHz, virtually no change from the "worst case" position.

(In your dreams, and wrong as usual. You obviously don't have the correct thevenin circuit. Did you even bother to measure it? With the 100k grid resistor, the pot does make less difference, but 6 to 10k isn't much difference.

There are major differences between midpoint and control set to "max".
It depends on getting rid of the 100k grid resistor as mentioned indicated above, and then the frequency response is determined by the source Z (when when the pot is at max volume, wiper arm 100k from ground) and the capacitance. This raises the frequency response well above that of midresistance point, which is stated in my post. And remember, the FR deviation accumulates from circuit to circuit.)

I actually choose the circuit and recommended precisely because of the way it bahaves WRT to capacitive loading on the volume control, plus it's comparably low output imedance without using cathode followers outside a feedback loop. And of course because keeping the feedback loop around one stage only minimises the blatant sonic problems caused by using loop feedback around muktiple stages.

(Compromise as usual instead of using a good design to begin with.
1st, capacitance does nothing good in this area.

2ndly, one doesn't need cathode followers, another stage, or the circuit you showed. There are ways around both.

3rdly, Using feedback around one stage. The problems of feedback depends on what follows, ICs capacitances, input Z of amp etc.
Do you even understand how these relate???

It makes a pretty good linestage, both subjectively and objectively, though I personaly would prefer an S&B TX-102, against which it can be easily compared BTW....

(Hope you don't give him a crappy PS design. This one measures and sounds crappy, if you like the highs dissappearing.)

_______________________________________

Yes, the gain does change the miller capacitance, but not enough to raise the poor frequency response of the network out of the audio band. Halving the gain to 5 only doubles the FR; thus still very poor.

And time delays in the feedback. This time delay depends on the Capacitance of ICs, and tube input capacitance of the amp etc. Changing Z input of the amp, changes feedback ratio etc., which changes the miller capacitance etc.

Mediocre design.

thoriated

20th March 2004 07:14 PM

Quote:

And this for the linestage.

Hi, Ashok -

Can you lose C1? It doesn't seem to be adding that much and I'm personally allergic to 'lytics in the signal path:). Perhaps it might be advantageous if C3 could be made larger to minimize lf phase shift and rolloff, also.

ThorstenL

20th March 2004 08:36 PM

Re: Wow

Konnichiwa.

Quote:

Originally posted by Positron (It is an engineering program, you are way off, either in calculating or the thevenin circuit itself.)

Hmmm. My results come from P-Spice and cloesely match those I observed on a real version of this stage.

Quote:

Originally posted by Positron Last string I mentioned there is a whole world you don't understand. You just demonstrated it again.

Last time I mentioned that you deliberatly claim unrealistic results and consitently fail to show an appreciation of basic electronics. I again observe the same thing.

Quote:

Originally posted by Positron You better check your math again, or get the thevenin circuit correct.

My math for the Circuit I presented is correct. Please build teh circuit if you do not believe basic electronics and physical laws hold sway, you will be thought that still do.

Quote:

Originally posted by Positron I also didn't include the capacitance of the volume control itself either.

Nor did I, however most modern volume controls show very little local capacitance (mostly because the are largely made from plastic.

Quote:

Originally posted by Positron In your dreams, and wrong as usual.

How would you know, as you obviously fail to even appreciate how the circuit works.

Quote:

Originally posted by Positron There are major differences between midpoint and control set to "max".

Sorry, NOT IN MY CIRCUIT.

Quote:

Originally posted by Positron It depends on getting rid of the 100k grid resistor as mentioned indicated above, and then the frequency response is determined by the source Z (when when the pot is at max volume, wiper arm 100k from ground) and the capacitance. This raises the frequency response well above that of midresistance point, which is stated in my post.

Non of which has anything to do with the circuit I presented. So we are again at the game of using numbers that while having some relation to the context are completly unrelated to the topic at hand. Well, nothing new.

Quote:

Originally posted by Positron Compromise as usual instead of using a good design to begin with.

The compromise was set by the basic requirements stated. We may argue if my circuit works better in reality, within the stated limitation than yours - oopps, I forgot, you don't suggest circuits, you only try to put other peoples suggestions down without even comprehending what is being suggested.....

Quote:

Originally posted by Positron Using feedback around one stage. The problems of feedback depends on what follows, ICs capacitances, input Z of amp etc.
Do you even understand how these relate???

Absolutely.

Quote:

Originally posted by Positron Hope you don't give him a crappy PS design. This one measures and sounds crappy, if you like the highs dissappearing.

So, between my posting the circuit (which was previously build with ECC88 and a slightly higher gain but also tested with the 6CG7 which is a noval 6SN7) you build the circuit, managed to have an exhaustive listen, to test the crcuit on the bench and posted your results. All within 5 Hours. I am mightly impressed. I cannot compete with that. However, when you build the ciorcuit you must have made a few wiring mistakes (not to worry, even happens to me when I'm rushed) and as a result your tests seem to have gone a little astray.

Or maybe you did not test the circuit? Maybe you just used what little of electronics to try to understand and abysimally failed to do so?

Quote:

Originally posted by Positron Mediocre design.

Well, why don't you suggest one that is demonstrably better? I don't mind. This is a free for all. But please ensure to provide sufficient wide bandwidth, low output impedance and distortion. It's not like I'm stopping you to present your designs, which are so superior to uthose from the other great unwashed.

Sayonara

thoriated

20th March 2004 08:38 PM

Quote:

It makes a pretty good linestage, both subjectively and objectively, though I personaly would prefer an S&B TX-102, against which it can be easily compared BTW....

Can you expand a bit on the comparative sonic advantages of each relative to the other? The TX-102 seems to be a bit pricy, maybe justifying a little tweaking of the non-magnetic option.

TIA -

ThorstenL

20th March 2004 09:04 PM

Konnichiwa,

Quote:

Originally posted by thoriated Can you expand a bit on the comparative sonic advantages of each relative to the other? The TX-102 seems to be a bit pricy, maybe justifying a little tweaking of the non-magnetic option.

quote:
Originally posted by Positron
(It is an engineering program, you are way off, either in calculating or the thevenin circuit itself.)

Hmmm. My results come from P-Spice and cloesely match those I observed on a real version of this stage.

<Really, well you didn't use the proper thevenin circuit did you.>

quote:
Originally posted by Positron
Last string I mentioned there is a whole world you don't understand. You just demonstrated it again.

Last time I mentioned that you deliberatly claim unrealistic results and consitently fail to show an appreciation of basic electronics. I again observe the same thing.

<Interesting as I am the one who graduated number 1 in my Electronics Engineering class and you don't have a degree, do you???? Of course, you skipped over that one last string, didn't you. And you lost pitifully last string, remember.
Caught you stating you had Way more than two years into tubes, but your own bio stated you had only approx 2, remember? You've have been caught numerous times posting inaccurate positions I made, remember. Isn't that called lying?
By the way, do you know some more of the other weaknesses of "your" circuit? If you can't answer this, then who is the novice?>

quote:
Originally posted by Positron
You better check your math again, or get the thevenin circuit correct.

My math for the Circuit I presented is correct. Please build teh circuit if you do not believe basic electronics and physical laws hold sway, you will be thought that still do.

<Well, obviously, then you don't know the BASICs of how to thevenize a circuit. And I am the novice? >

quote:
Originally posted by Positron
I also didn't include the capacitance of the volume control itself either.

Nor did I, however most modern volume controls show very little local capacitance (mostly because the are largely made from plastic.

<Where did you come up with that reply? If you had measured some famous controls, the capacitance is fairly large do to the shielding of the metal case. Caught yourself again.>

quote:
Originally posted by Positron
In your dreams, and wrong as usual.

How would you know, as you obviously fail to even appreciate how the circuit works.

<Caught yourself again. And who pointed out the major weaknesses of the circuit? Me. Can you explain the ones I haven't brought up yet? This will tell if you really understand your circuit.>

quote:
Originally posted by Positron
There are major differences between midpoint and control set to "max".

Sorry, NOT IN MY CIRCUIT.

<Unfortunately, there is.>

quote:
Originally posted by Positron
It depends on getting rid of the 100k grid resistor as mentioned indicated above, and then the frequency response is determined by the source Z (when the pot is at max volume, wiper arm 100k from ground) and the capacitance. This raises the frequency response well above that of midresistance point, which is stated in my post.

Non of which has anything to do with the circuit I presented. So we are again at the game of using numbers that while having some relation to the context are completly unrelated to the topic at hand. Well, nothing new.

<If you understood the circuit as you claim, you would have answered differently.
At midpoint of the pot, phase shifting occurs well below 20khz. It is easily heard on a system.>

quote:
Originally posted by Positron
Compromise as usual instead of using a good design to begin with.

The compromise was set by the basic requirements stated. We may argue if my circuit works better in reality, within the stated limitation than yours - oopps, I forgot, you don't suggest circuits, you only try to put other peoples suggestions down without even comprehending what is being suggested.....

<No, there are many circuits I don't comment on here. But when I see a completely idiotic circuit, and the person wants something really good, well.... Let's see if you can find some more problems with this circuit. By the way, one can build much better with his list of parts, wider bandwidth, very low distortion, low output Z.>

quote:
Originally posted by Positron
Using feedback around one stage. The problems of feedback depends on what follows, ICs capacitances, input Z of amp etc.
Do you even understand how these relate???

Absolutely.

<When were you going to tell him about the problems I have mentioned and others I haven't yet?> Can you explain more other than "absolutely"?>

quote:
Originally posted by Positron
Hope you don't give him a crappy PS design. This one measures and sounds crappy, if you like the highs dissappearing.

So, between my posting the circuit (which was previously build with ECC88 and a slightly higher gain but also tested with the 6CG7 which is a noval 6SN7) you build the circuit, managed to have an exhaustive listen, to test the crcuit on the bench and posted your results. All within 5 Hours. I am mightly impressed. I cannot compete with that. However, when you build the ciorcuit you must have made a few wiring mistakes (not to worry, even happens to me when I'm rushed) and as a result your tests seem to have gone a little astray.

<Typical response. I have been designing for decades. I have used 6sn7a etc in all sorts of designs, including the one you presented, and have some in my stash right now.>

Or maybe you did not test the circuit? Maybe you just used what little of electronics to try to understand and abysimally failed to do so?

<Well, if you understood the circuit, and how it sounds, as you claim, you would have known all the weaknesses and flaws and could have recommended a better design. (Again, if you understand the basics, you explain the other problems this circuit has, that I have not yet mentioned. If you don't, everyone will know your novice status.)>

quote:
Originally posted by Positron
Mediocre design.

Well, why don't you suggest one that is demonstrably better? I don't mind. This is a free for all. But please ensure to provide sufficient wide bandwidth, low output impedance and distortion. It's not like I'm stopping you to present your designs, which are so superior to uthose from the other great unwashed."

<Sure, I am just going to give you my designs, which took years to develop. typical shill comment. :whazzat: >

<Ok, keep away from CFs, work much more on the power supplies, and use low output Z, high bandwidth, low distortion tubes, the best brands sonically too.>

ThorstenL

21st March 2004 11:58 AM

Valves, Millercapacitance and feedback circuits

Konnichiwa,

Well, it seems I made my usual error and actually assume that those who claim to understand electronics are sufficiently competent not talk complete loblocks (as they say on Planet anagramia) and hence will make some sense and the rest doesn't want to know anyway. Seems I'm wrong.

So hence, let's evaluate, strictly on paper and in a simplified form the 6SN7 parallel linestage.

First, open loop we have an anode impedance of around 8k. Paralleled this gives an internal impedance of 4k and with a 10k Load a stage gain of 12.6, this comes from comparing the Load (22k//10k) Rl and the internal impedance of the stage (4k) Ri as voltage divider and reducing the Valves Mu by this factor as:

Gain = Mu/((Rl+Ri)/Rl)

Now we can calculate the Miller capacitance and using this also the input capacitance. For average Valves my Datasheets list 3.3 & 3.5pF as Anode-Grid Capacitance, so both systems in parallel show 6.8pF which will be "amplified" by the stage gain, 12.6 to give a virtual 85pF input Capacitance between Grid and Andode. Adding the other parasitic valve capacitances we get a rounded 100pF capacitance if good quality (ceramic) sockets are used and the layout is sensible.

Now we can calculate the relative impedance of the gridnode to obtain the OPEN LOOP bandwidth (note that this a looped feedback amplifirer stage).

At the worst case (volume control at -6db setting) the 100k Volume control will present around 25k output impedance, to this is added the 100k Input resistor of the inverting, looped feedback amplifier. In parallel to this overall 125k impedance is the feedback loop and internal impedance of the circuit, so basically 244k. In parallel this gives a impedance of 82.6k.

If we combine the 82.6k and the 100pF we get a timeconstant of 8.26uS or a -3db point of 159115/8.26=20KHz.

So, OPEN LOOP we have a -3db point of 20KHz in the worst case scenario. This frequency is shifted upwards in the usual manner as discussed by Harold Black in the 1930's by the diference between the Open Loop gan and the closed loop gain.

In my case the closed loop gain is 2, so we have around 6.3 or 16db feedback factor, so in theory and not accounting for some of the parasitics I include in my P-Spice simulation (such as the 1nF output load of the 10k/1nF IEC Load) we have theoretically a closed loop -3db point of 126KHz under worst case conditions, but only if there is no capacitive load. My simulations assume 1nF capacitive load and hence are less favourable.

Let's also work out the apparent output impedance of the Linestage. WE have a 22k Load in parallel with a 4k internal impedance, giving under open loop conditions 3.4KOhm output impednace. This will be reduced (again) by the feedback factor giving us theoretically around 530 Ohm output impedance.

A last check would be how much the difference between fully open and worst case setting on the volume control is. Using above Methode we now have 100k//244k to give 71k effective impedance at the grid node. This gives a 7.1uS timeconstant or 22.4KHz open loop -3db point and closed loop theoretically a 141KHz -3db closed loop.

This concludes the current EE101 lesson for those who wanted to know and those who obviously slept through EE101. We thank you for your attention and suggest that everyone her or himself evaluate designs themselves, using sensible methodes instead of listening to people who obviously lack basic analytic skills (or deliberatly distort the truth - I leave it to the audience to decide which takes place).